This invention relates to a small power plant utilizing waste heat, and particularly concerned with said power plant characterized by having an intermediate heat transfer medium between a heat source and a turbine operating medium so as to heat said turbine operating medium through direct contact.
In general, freon turbine power plants are being used as the small power plants utilizing waste heat. With said freon turbine power plant, freon has the upper working temperature limit as viewed from heat stability and corrosion of freon, and the temperature of freon at the outlet of boiler must be controlled to less than said upper working temperature.
However, in said freon boiler, the temperatures of the heating medium and side heating medium such as a gas and a steam can not be elevated to too much higher temperature than the upper working temperature limit of freon in order to prevent degradation of freon by heat (at about 170.degree. C.) which is being in contact with heat conductive pipes of boiler. The reason is that degradation of freon by heat results in generation of chlorine which is highly corrosive. For this reason, the difference in temperature between freon gas at the outlet of boiler and the heating medium can not be made too large. Accordingly, the use of a freon boiler entails an increase in the size as compared with the output. Further, freon has such a property that it has a strong affinity for oil, and hence freon melts oils such as a turbine oil for bearings of freon turbine with the resultant decrease in the capacity of said oil in lubricating the bearings.